The fossil of a hemithecellid, which was probably a segmented mollusk, can appear as plates or valves that are elongate, arrow-head, or tapering in form. The valves of these mollusks vary in shape and ornamentation. Some paleontologists classify these early Paleozoic "plated mollusks" as primitive chitons (amphinuerans). Others consider them representatives of an extinct molluscan class (a category based on body plan). Consequently, differing views exist as to what the living animal looked like. An animal covered with eight more-or-less equal-sized plates is one model. Another possibility is an elongated animal made up of sixteen plates.
Monoplacophorans represent a relatively new (since 1950) class of mollusks. Specimens are usually represented by a partially segmented animal covered by a cap or cone-shaped shell. Representatives of this molluscan class are some of the most diverse and abundant of Ozark Cambrian and Lower Ordovician fossils.
A monoplacophoran genus with an uncurved, cone-shaped shell. Like most early Paleozoic monoplacophorans, Kirengella is usually associated with stromatolite reefs.
A spoon-shaped monoplacophoran, sometimes common in cherts weathered from strata of the Upper Cambrian and Lower Ordovician of the Ozarks. Proplinid monoplacophorans often exhibit eight muscle scars arranged in a horseshoe-shaped pattern. Some proplinid monoplacophorans can get quite large.
Monoplacophorans with a recurved shell includes this Ozark genus as well as Hypseloconus, one of the most widespread of Cambrian monoplacs. Recurved (Hypseloconid) monoplacophorans usually occur associated with stromatolite reefs. An unusual occurrence of hypseloconid monoplacs occurs associated in a sandstone matrix, the sandstone being embedded between boulders of Late Precambrian, Keewanawan basalt boulders at St. Croix Falls, Minn., on the edge of the Canadian shield.
Proplina with Muscle Scars
Monoplacophoran genus Proplina showing muscle scars. The presence of multiple muscle scars like those shown here, is what distinguishes monoplacophorans from cap-shaped gastropods. Multiple muscle scars of this type are associated with segmentation of the animal itself, a primitive feature in mollusks, mollusks normally being non-segmented. Living monoplacophorans, in the 1950's were discovered living in a deep sea trench.
Trilobites were one of the most abundant and dominant animals of the early Paleozoic. Trilobites were marine arthropods which diversified (speciated) into a great variety of types (species). Trilobite fossils can be locally abundant in Ozark rocks, but most specimens are incomplete. A large number of trilobite genera and species are unique to the Ozarks.
A number of snail-like shells occur in Cambrian rocks of the Ozarks. They have a number of features which are peculiar and different from the shells of undoubted marine gastropods. The snail-like fossils may not represent true snails or gastropods. These forms usually have left-handed coiling (sinistrial), whereas true gastropods usually coil in a clockwise (dextral) direction. Some of these snail-like forms (Bellerophontids) coil along a flat plane, and others can coil in either a right- or left-handed direction. These fossils may represent an extinct branch (or branches) of gastropods. Alternately, they may demonstrate instances of independent molluscan evolution, where they record extinct molluscan classes or what might be considered as failed evolutionary "experiments".
Matthevia is a problematic fossil first named by Charles D. Walcott, the discoverer of the incredible Cambrian fossils of the Burgess Shale. Like other Cambrian and Early Ordovician plated mollusks, Matthevia has been considered as a primitive member of the class Amphinuera (chitons), or is thought to represent an extinct class of mollusk. The single valve of Matthevia has two "pockets" which, on internal fossil molds, show up as two opposite-facing cones. Most of the Matthevia occurrences in New York, Utah, and Nevada have these pockets or "cones" of unequal size. However, Cambrian specimens from the Ozarks usually have the two equal-size pockets with bilaterally-symmetrical valves. Lower Ordovician Matthevia specimens of the Ozarks have the two different-size pockets.
Stromatolites are fossils which show the life processes of cyanobacteria (formerly called blue-green algae). The primitive cells (Prokaryotic type), lived in huge masses that could form floating mats or extensive reefs. Masses of cyanobacteria on the sea floor deposited calcium carbonate in layers or domes. These layered deposits, which have a distinctive "signature" are called laminar stromatolites. This is an example of a layered stromatolite from the Ozark Precambrian. Most often, stromatolites are appear as variously-sized arches, spheres, or domes. Ozarkcollenia, a distinctive type of layered Precambrian stromatolite, pushes the appearance of life in the Ozarks to well over a billion and a half years ago.
Aglaspids are arthropods related to, or resembling, modern horseshoe crabs. They are known exclusively from the Lower Paleozoic, primarily the Cambrian Period. A spike-like tail, called a telson, is one of numerous features which separates them from the trilobites. Trackways given the name Protichnites may have been made by aglaspids or aglaspid-like animals.
Trackways of a multi-legged animal with a "tail" located down the middle are yielded by some of the slabby Cambrian sandstones in the Ozarks. The maker of these trackways must have been an animal much like the present horseshoe crab. The sandstone slabs where Protichnites trackways occur, show dessication. This suggests that the tracks may have been made on a sand flat above the tide, rather than underwater. The animal which made Protichnites tracks could have been one of the Earth's first air-breathing animals. Cambrian land life is almost unknown in the fossil record. Presumably, there was little life on land at that time; life in the Cambrian was almost exclusively marine.
Conodia is one of a number of enigmatic Ozark fossils. Arrowhead-shaped plates with a ball-and-socket termination represent one type of what are called multi-plated-mollusks. Some paleontologists consider these multi-plated-mollusks (which includes Conodia, Hemithecella, Matthevia, and Robustum) as representing primitive amphineurans (chitons). If these forms are chitons, they would have been composed of eight valves. Some evidence suggests that they had more than that number, other evidence suggest that they may have been covered by a single shell.
Paleoloricates are presumed multi-plated mollusks considered by some to be primitive chitons or amphinurans. If chitons, they would have had eight valves. No complete (articulated) specimens have been found, and it is a possibility that a single shell may have covered the animal. Some Ozark paleoloricates exhibit a pattern of muscle scars, suggesting affinity to monoplacophorans. Configuration of these valves and how they covered the actual animal when living is controversial and unclear
Today, cephalopods are one of the most highly evolved molluscan classes. They also represent, in the form of the octopus, one of the most highly-evolved and complex invertebrates. In the early Paleozoic, the soft cephalopod body mass occupied an elongated, coiled, or cone-shaped shell. Some of the earliest cephalopod fossils known are found in the Ozarks.
Example of the tracks and a reconstruction of what Climactichnites possibly might have looked like. Sandstone beds of the Ozark Cambrian can contain the trackways of a variety of puzzling animals, one of which is has been given the name Climactichnites. Trackways of Climactichnites appear to have been made by a large, fleshy animal advancing over sand flats during high tide. They resemble tracks made by a motorcycle
Oval or cone-shaped, elongate shells are common fossils in Cambrian age strata. Sometimes hyolithes exhibit on very well-preserved specimens, what looks like a "lid" and two supporting "guide wires" (helens). Like many Cambrian fossils, hyolithes are a paleontologic enigma! Some paleontologists classify them as representatives of an extinct Molluscan class, others consider them as members of an extinct phylum. Hyolithes often occur in clusters on bedding planes of Cambrian shale or as groups of shells in limestone or dolostone.
Late Paleozoic (Mississippian) specimen of Conocardium which occurs in the outliers of Mississippian age of the Ozarks. Rostrochonchs are bivalve shells in which the two valves are not hinged as they are in the mollusk class Pelecypoda (clams). Rostrochonchs are considered as representatives of an extinct molluscan class. Like many extinct high level taxa, rostrochonchs are strictly Paleozoic mollusks. Late Paleozoic forms such as the genus Conocardium have sometimes been placed with the brachiopods or with the Pelecypods. They are in the reference work Index Fossils of North America under Pelecypods.
The amphinuerans are a group (class) of multi-plated mollusks which exhibit eight shells (called valves). Between the head and the tail valves are six smaller ones. In fossils, these valves are usually disarticulated and scattered. A number of different types of single valves are found in Late Cambrian rocks which are attributed to chitons, and many of these have come from the Ozarks. Some of these valves exhibit muscle scars identical to those found on monoplacophorans. The affinity of these and many other "plated mollusks" such as "paleoloricate amphinuerans" to the amphinuerans is unclear. These early "multiplated" mollusks may also have had greater or less than eight valves.
These plated mollusks consist of thick, bead-like plates or valves which, on fossil molds, resemble small saddles. A few of the species of Robustum exhibit a somewhat pointed end, suggestive of the pointed end of hemithecellids. Like other examples of multi-plated-mollusks, there is disagreement as to the number of valves on the Robustum animal and also as to whether Robustum really was multi-valved.
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Cambrian fossils - Ordovician fossils - Missouri